Spinal fixation has become a common approach in treating spinal disorders, fractures, and for fusion of vertebrae. A common device used for spinal fixation is a bone fixation plate. A typical bone fixation plate includes a relatively flat, rectangular plate having a plurality of apertures therethrough. A corresponding plurality of fasteners, i.e., bone screws, are provided to secure the bone fixation plate to a bone, such as two adjacent spinal vertebrae. The screws are firmly tightened to secure the bone fixation plate to the bone or bones to be fixed. Typical examples of such bone fixation plates are illustrated in U.S. Pat. No. 5,364,399, issued Nov. 15, 1994, to Lowery et al. and U.S. Pat. No. 5,601,533, issued Feb. 11, 1997, to Trebing et al. These patents are cited as being exemplary of the general state of the art with regard to bone fixation plate technology. In general, these types of devices can be utilized for anterior fixation of the spine for cervical, lumbar, and/or thoracic fixation.
The basis of anterior fixation or plating is to approach the spine from anterior or anterio-lateral approach and use the screws to solidly mount the bone fixation plate to the affected vertebrae. This approach is commonly used in cases of trauma, tumors, and degenerative conditions. Often, in addition to the application of a bone fixation plate, graft material may be combined in attempt to permanently fuse together adjacent vertebrae. The graft material can consist of bone grafts obtained from other bones in the patients body or from cadaver bones.
A common problem associated with the use of such bone fixation plates is the tendency of the bone screws to "back out" or pull away from the bone into which they were fixed. This problem occurs, primarily, due to the normal motion of the body and spine. The spine is a very dynamic entity and is constantly moving. This problem is especially prevalent in areas of high stress such as the spine. This is a particularly important problem because as the screw becomes loose and pulls away from the bone, the head of the screw can rise above the surface of the bone fixation plate and, possibly, even work its way completely out of the bone. This condition can create a number of potentially serious problems given the number and proximity of blood vessels and other critical structures near the locations of anterior spinal plate fixation.
A number of various designs have been brought forth in attempts to prevent screws from pulling away from the bone and/or to prevent the screws from backing out or pulling away from the surface of the bone fixation plate. For example, the Lowery et al. patent, discussed above, discloses an anterior cervical plating system incorporating a locking screw which engages the heads of the bone screws used to secure the cervical plate to the vertebrae. The locking screw is positioned above the bone screws and is driven against the heads of the bone screws to and rigidly fix the bone screws to the plate. However, for this locking mechanism to work, the distance between the heads of the bone screws must be kept to a minimum, thereby limiting the potential applications of the bone fixation plate. Additionally, while the Lowery et al. patent allows for the bone screws to be angled, if the screws are not angled exactly the same amount, which is very difficult to achieve, the locking screw cannot adequately contact both bone screw heads.
Another example of a mechanism for preventing bone fixation screws from backing out or becoming dislodged from the bone is set forth in the Trebing et al. patent discussed above. The Trebing et al. patent discloses a mechanism whereby the bone fixation plate is threaded and is used in combination with a bone screw having both bone engaging threads and a threaded portion near the head of the bone screw which is complimentary to the threaded hole in the bone fixation plate. In this mechanism, the screw is rigidly fixed to the bone fixation plate. However, it is possible to lock the bone screw to the bone fixation plate while leaving a gap between the bone fixation plate and the bone. This problem can cause inferior fixation of the bone or even total failure of the fixation.
Various other mechanisms used to prevent bone screws from pulling out of bones include cams which engage and lock the screws and the use of expanding head screws which expand outwardly when adequate force is applied thereto to engage the holes in the bone fixation plate. All of these particular designs have drawbacks including potential for breakage or requiring particular precision and alignment in their application in order to correctly work.
Yet another apparatus for preventing bone screw back-out from a bone fixation plate is shown in U.S. Pat. No. 5,578,034, issued Nov. 26, 1996, to Estes. The Estes patent discloses a system for internal fixation of bone which includes a bone fixation plate having a number of bores therethrough, a corresponding number of screws each having an enlarged head portion and an elongated shaft portion defining bone engaging threads thereabout and a non-threaded portion between the head and the threaded portions, and a corresponding number of screw anti-backout members each having a bore therethrough. The screw anti-backout members are inserts positioned within the bores of the fixation plate and are initially sized to slidingly receive an elongated screw shaft therethrough. During application of the fixation plate, the bone screws are advanced through the bone fixation plate bores and the screw anti-backout members which are positioned within the plate bores to screw the bone fixation plate to the underlying bone. Thereafter, the apparatus is sufficiently heated to shrink the bores of the screw anti-backout members, thereby trapping the non-threaded portion of the screw shafts located between the fixation plate and the threaded portions. The anti-backout collars are immobilized within the bore of the fixation plate. Thus, the collar and fixation plate remain in fixed relationship to each other after fixation to the underlying bone.
Other types of inserts or collars have been used with bone fixation plates for a variety of reasons such as those shown in U.S. Pat. No. 4,388,921, issued Jun. 21, 1983, to Sutter et al. and U.S. Pat. No. 5,607,428, issued Mar. 4, 1997, to Lin. Sutter et al. discloses a bone fixation plate in which sleeves are placed in openings provided in a bone fixation plate. A screw is placed through the sleeve and into the underlying bone. By tightening the screw, the sleeve is clamped in place with relation to the bone fixation plate thus assuring that the fixation plate will stay rigidly connected with the screws.
The Lin patent discloses a bone fixation plate having a direction adjusting ring disposed in at least one hole in the fixation plate. Upon insertion and tightening of the threaded bone screw, arresting edges of the direction adjusting ring are urged into engagement within the hole to securely fix and retain the direction adjusting ring therein.
Both the Sutter et al. and Lin patents disclose, similar to those patents described above, an insert that is rigidly disposed or fixed in a hole or aperture in the fixation plate. They do not allow for movement of the insert with relation to the bone fixation plate.
Therefore, it would be desirable to provide a bone fixation assembly which would allow positive, rigid fixation of a bone fixation plate to a bone, such as adjacent vertebrae, while allowing movement, stress, or dynamic load sharing of the adjacent vertebrae thereby enhancing the bone rebuilding process and enhancing the success of a bone graft. Additionally, it would be desirable to have a bone fixation plate assembly which would prevent the application of high stress on the screws and plate which would lead to failure of the assembly, including the actual breakage of the screws and closure of the previously formed gap between the adjacent vertebrae. Additionally, it would also be desirable to provide a bone fixation plate assembly that allows the fixation screws to be locked to the plate to prevent the screws from backing out of the plate while allowing the plate to be firmly seated against the underlying bone. Further, it would desirable to provide a bone fixation plate assembly which requires no small parts nor requires no additional steps to lock the screws to the bone fixation plate.